The present invention relates to a heat-shrinkable laminate film comprising a core layer of a copolymer of vinylidene chloride, two surface layers of polyamide(s) and layers of an adhesive between each of the two surface layers and the core layer and a process for producing the heat-shrinkable laminate film.
The adhesive is selected from the group consisting of a modified copolymer obtained by grafting a copolymer of ethylene and a vinyl carboxylate or an acrylic ester with an ethylenically unsaturated carboxylic acid or an acid anhydride thereof and a polymeric material obtained by neutralizing said modified copolymer with a metallic compound or a thermoplastic polyurethane.
The heat-shrinkable laminate film shows rates of shrinkage not less than 10% both lengthwise and breadthwise after keeping the film 3 seconds in an atmosphere of 90.degree. C, rates of delayed recovery of not more than 2% lengthwise and breadthwise at room temperature and the rate of creep of not more than 20% after immersing the film 10 seconds in a warm water of 80.degree. C with a load of 1 kg/mm.sup.2.
So far, meat products such as hams and sausages are filled and/or packaged in a thermoplastic casing and subjected to heat treatment for a few minutes to a few hours at a temperature in the range of about 70 to 95.degree. C and offered to markets.
The important points which should be satisfied by the packaging material used for packaging such meat products are as follows:
(1) The casing must have a sufficient impermeable property against gaseous oxygen in order to prevent the content to be spoiled during the period of distribution.
(2) The jelly of meat juice, etc. should not deposite between the casing and the content, and particularly, when the content is a meat product such as ham, sausage, etc., it is required that the casing firmly adhere to the content.
(3) Since the wrinkled or pleated appearance of a packaged foods gives an impression that such a food is a stale product, it is not desirable that the casing becomes wrinkled.
(4) The casing should not be broken nor deformed from a cylindrical form by a pressure at the time of filling (generally 0.2 to 0.5 kg/cm.sup.2 G) and a pressure due to an expansion of the content at the time of heat-treatment. Namely, to the casing, an anti-creeping property at a high temperature is required. For instance, it is required that the casing is not deformed by the stress of at least 0.2 to 0.3 kg/mm.sup.2 at the time of heat-treatment at a temperature of 70 to 95.degree. C.
(5) It is desirable that the casing has a sufficient flexibility not to have any pinhole at a time of filling.
In order to respond the above requirements, although an unstretched monolayer casing comprising a homopolyamide (Nylons 11 and 12) and a coextruded two layer casing having a inner layer of the homopolyamide and an outer layer of Nylon 6 have already been publicly known, these casings are apt to be deformed by a filling pressure and since they are unstretched films, he wrinkles generate remarkably at the time of cooling after heat-treatment.
On the other hand, a heat-shrinkable casing comprising a polyamide or a mixture of the polyamide and a polyolefin has been proposed in Japanese Patent Application Laid-Open (KOKAI) No. 55-74744/1980 and it is shown therein that a deformation of the casing and a generation of wrinkles on the casing has been prevented. Furthermore, the casing comprising a thermoplastic polyester (for instance, polyethylene terephthalate) has also been known. However, since the above casings do not have the sufficient impermeability to gaseous oxygen and water vapor, they have common defect that the period of storage for the food products is unavoidably shortened. Besides, there is a disadvantage that the polyester casing does not firmly and closely adhere to the content such as ham, sausage, etc.
While, the casing comprising a copolymer of vinylidene chloride having a barrier property to gaseous oxygen and water vapor has already been widely used in the market, however, not only the extent of the close adherence of these casings to ham and sausage is insufficient but also the cylindrical forms of these casings are deformed sometimes by their dead weights at the time of heat-treatment when a size of the casings is large, and insufficiencies of their anti-creep property at a high temperature are also pointed out. Furthermore, pinholes are apt to be formed on the casings at the time of filling, and accordingly, improvements of such properties have been required.
In order to solve these defects, a heat-shrinkable laminate film comprising a copolymer of vinylidene chloride and a polyolefin and/or a polyamide has been proposed in Japanese Patent Applications Laid-Open (KOKAI) Nos. 59-79753/1984 and 59-174350/1984 and U.S. Pat. No. 4,112,181.
However, the films disclosed in Japanese Patent Applications Laid-Open (KOKAI) Nos. 59-79753/1984 and 59-174350/1984 are apt to be curled and easily generate undesirable deformations such as delayed recovery and creeps in warm water, accordingly, it cannot be said that the films have a sufficient dimensional stability. Besides, in the laminate films disclosed in U.S. Patent No. 4,112,181, adhesions between layers are weak and accordingly, the laminate film has a defect of peeling off between the layers. In order to avoid the defects, layers of an adhesive are used for firmly adhering a copolymer of the core layer to a polyamide of the both surface layer. As the adhesive, a mixture of a copolymer of ethylene and vinyl acetate and a copolymer of ethylene and acrylic acid (refer to Japanese Patent Application Laid-Open (KOKAI) No. 49-41476/1974) and a mixture of a copolymer of ethylene and vinyl acetate and a copolymer of ethylene and ethyl acrylate (refer to Japanese Patent Application Laid-Open (KOKAI) No. 51-119075/1976) have been known.
However, the process for producing a heat-shrinkable film by biaxially stretching a coextruded laminate film, as in the present invention, necessitate an excellent adhesion between the layers against a peeling-off power at the time of stretching and it is desirable that the adhesive layers are heat-resistant in a range of heat treatment temperature (usually from a few minutes to a few hours at 70 to 95.degree. C) to prevent peeling off at the treatment.
By the conventional adhesive, a sufficiently satisfiable result has not been obtained in the production of the heat-shrinkable laminate film which is the object of the present invention. Besides, in a case of conventional stretching of a single film, a laminate film or a sheet comprising a polyamide, an extremely high technique has been necessary, because the polyamide has hydrogen bonds.
Generally, to make a biaxially stretched single or laminate polyamide film, the film extruded from a T-die in a plate-form is biaxially stretched by a tenter-system. However, in order to produce a film for filling and/or packaging hams and sausages, it is desirable to extrude the film in a tubular form. Although the film which has been extruded in a tubular form is stretched generally by an inflation system, since a large stretching force is necessary when a polyamide film is used, the polyamide film obtained by an inflation system is easy to be non-uniform in thickness and it has been difficult to industrially and effectively manufacture a film of a uniform thickness. Furthermore, it has been pointed out that the adhesion between the layers becomes weak when the laminate film is stretched.
Taking into consideration the above situation, the present inventors have studied to provide heat-shrinkable laminate films which overcome the defects of conventional one and is suitable to package foodstuffs and have found that by coextruding a copolymer of vinylidene chloride as a core layer, a polyamide resin as two surface layers and an adhesive as adhesive layers between each of other layers, and by biaxially stretching the coextruded film under certain conditions, the heat-shrinkable laminate film excellent in a barrier property to gaseous oxygen, a barrier property to water vapor, an anti-creep property at a high temperature, an anti-pinhole property, a closely adhering property to meat, hygienic point of view, an anti-curl property, an anti-creep property in warm water, an anti-delayed recovery property and an anti-peeling off property is to be obtained The appropriate adhesive is the one selected from the group consisting of a modified copolymer obtained by grafting a copolymer of ethylene and a vinyl carboxylate or an acrylic ester with an ethylenically unsaturated carboxylic acid or an acid anhydride thereof and a polymeric material obtained by neutralizing said modified copolymer with a metallic compound or a thermoplastic polyurethane. The necessary biaxially stretching conditions are (1) cooling the coextruded film rapidly to a temperature which is lower than the second order transition temperature of the polyamide (if two different kinds of polyamide are used, the lower secondary transition temperature of the two) and is not lower than 30.degree. C of the transition temperature; (2) making the layer of a copolymer of vinylidene chloride in an amorphous state; (3) making both surface layers of polyamide absorb water 1 to 5% by weight; (4) biaxially stretching the film at a temperature of 60.degree. C to 100.degree. C; and (5) heat-treating the stretched film for more than 2 seconds at 70 to 100.degree. C while making the outer surface layer of polyamide absorb water 2 to 7% thereby shrinking the film 1 to 15% both lengthwise and breadthwise.
Based on these findings, the present inventors have attained the present invention.